I have a curious problem that occurred with my 190E. First off the Fan Blower speed switch was jammed; so I replaced that with a new one from your parts department. Fan speed works as advertized now. However, if the Defrost button is pressed or either the airconditioning or max airconditioning button is depressed the motor will stall (with any fan speed selected).
If I increase RPM and attempt to turn on the air conditioning or the defroster, the RPM immediately drops and the engine stalls.
I am going friggin crazy here. I changed the .2 ohm resistor block and the A/C relay both to no avail. I need some help here.
By the way if the Defroster or A/C is left alone and any other combinations of fan speed or directional air or temp is selected all is normal. Need Help ASAP. Thanks.

I would check the compressor itself - is it locked-up. It shouldn't matter though, that's what the relay is there for to turn off the compressor if it's locked-up.

Try unplugging the compressor and see what happens, if the clutch is drawing too much amperage? Although this scenario should blow a fuse or the relay.

The only other thing that comes to mind would be pretty far fetched, POSSIBLY... the RPM signal being grounded/shorted when the compressor is engaged? This would definitely shut the engine down. But like I said that would be very strange indeed.

Thanks. I disconnected the electrical connector on the compressor and actuated the A/C, Max A/C and Defrost buttons and the engine idled on perfectly. Reconnected and tried again.....engine stalled. Seems I got a bad compressor clutch.
Do you agree? Curious that a relay or fuse doesnt blow preventing the engine from stalling. If this occurred at high speed my first concern would be loss of power steering. Please advise.
Regards,
DD

There is still a possibility that the compressor is locked up, and you have a belt that is good and tight with a lot of traction. Have you tried to turn the compressor by hand? Reach down the front of the engine (with the motor off of course ) and try to turn the part of the compressor clutch that is farthest forward, best to do this after the car has been sitting for several hours and is cool.

This will require a trip to your favorite MB tech to repair, and the expense of around $1000.00+. You will need to replace the compressor, receiver drier, and expansion valve, and flush the system to remove metal contamination. Mercedes recommends replacing the "manifold/muffler" hose assembly in the case of a compressor failure, which can be an expensive part. I always recommend using OE remaned compressors.

Then you will have to decide which refrigerant you will go back with, 134a or r12? I won't get into a debate on which is the way to go on this issue, but my opinion is for 134a retrofit for the difference in expense and future availability of product, JMO:p

Hope you are still breathing, sorry to have brought you to a bad conclusion on your problem, but at least you know.